Yes you can put Windows Vista on your PSP but it is just an offline website . Anyway you can still play some small games and you can use MSN Messenger on it. It also has some cool applications. If you want to put it on your PSP, here it is

put the pspWxp Folder into your Root, then go to your broswer, in the address bar type "file:/pspwxp/index.htm" then you are on Vista. You don't need Internet to get it, if your PSP can't go on Internet . Have fun!